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Style Guide

This document has been reviewed and discussed with @hsins.

For further reading, please see the definition of Convention over configuration in Wikipedia.

Disclaimer

General

  • In our coding convention, we use lowerCamelCase for both functions and variables. While this may deviate from the rules laid out in the Google C++ Style Guide and PEP 8 -- Style Guide for Python Code, it is done to maintain consistency with the LeetCode OJ system, which employs lowerCamelCase for over 99% of the time. Remember, the most important thing is to be consistent at all times.
  • In code snippets, import statements and brackets may be omitted for brevity (arguably), but they should be retained in real-world coding situations for clarity and maintainability.

C++

  • The code is only for demonstration and cannot be compiled.
  • Explicitly declaring types such as int, char, and string is often preferable over using the auto keyword introduced in C++ 11.
  • #include statements are omitted in code snippets for brevity.
  • Understanding the difference between size_t and int is crucial when traversing arrays in practical applications, as it can affect the outcome.
  • Qualifying with std:: prefix when accessing standard libraries is recommended for readability and maintainability in real-world coding situations, as it clearly indicates the source of the identifier being used.

Java

  • The code is only for demonstration and cannot be compiled.
  • Explicitly declaring types such as int, char, and String is often preferable over using the var keyword introduced Java 10.
  • import statements are omitted in code snippets for brevity.

Python

  • Private functions are prefixed with _, which may seem tedious. However, in the real world, we use tools like Mypy, Pytype, and Pyre for static type checking.
  • We pass the argument --indent-size=2 to autopep8 for a better viewing experience.
  • import statements are omitted in code snippets for brevity.

SQL

  • The naming of LeetCode is quite a mess and inconsistently uses plurals and singulars together. I'll use my best judgment to name tables in UpperCamelCase in the plural form and fields in lower_snake_case.
  • Proper indentation will be taken care of and treated seriously.
  • No puzzling table abbreviation.

Naming Cases

Rules

  • Class: UpperCamelCase
  • Function: lowerCamelCase
  • Variable: lowerCamelCase
  • Constant: kUpperCamelCase

Examples in C++

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// Class
class MyClass { ... }

// Function
function myFunction() { ... }

// Variable
int myVariable;

// Constant
constexpr int kMod = 1'000'000'007;

Template

We use C++ to demonstrate the idea, and the same concepts apply to Java and Python.

Rules

  • Declare the variables in the proper scope as slow as possible.

  • Declare constexprt as soon as possible.

  • Declare const variables as soon as possible.
  • Declare ans as soon as possible.

  • Since LeetCode is just an online judge system rather than a big project, we don't scatter all variables in different sections. However, we still sort the variables based on the time we first use each of them.

  • Code section (there should be one blank line between each sections.)

  • public

    1. boundary conditions
    2. initial variables
    3. There may be many kernels separated with one blank line, but there shouldn't be any blank line in each kernel.
    4. return

  • private

    1. private variables
    2. private functions

Schematic Template

We use C++ to demonstrate the idea, and the same concepts apply to Java and Python.

  • No blank lines between variables initialization.
  • Blank one single line between each section. However, if there's no sec 12, no blank line between sec 11 and sec 13.
  • If the last statement is not a paragraph (for loop most of the case), then no blank lines between it and the return statement.
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class Solution {
 public:
  func() {
    // (sec 0) boundary conditions

    // (sec 1) initial variables
    //   (sec 10) constexpr/const (size/length)
    //   (sec 11) ans
    //   (sec 12) declaration & operation
    //   (sec 13) purely declaration

    // (sec 2) kernels

    // (sec 3) modify original initial variables

    // (sec 4) kernels

    // (sec n) return
  }

 private:
  // private variables

  // private functions
  helper() { ... }

  anotherHelper() { ... }
};

Example (873. Length of Longest Fibonacci Subsequence):

  • code:
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class Solution {
 public:
  int lenLongestFibSubseq(vector<int>& arr) {
    const int n = arr.size();
    int ans = 0;
    vector<vector<int>> dp(n, vector<int>(n, 2));
    unordered_map<int, int> numToIndex;

    for (int i = 0; i < n; ++i)
      numToIndex[arr[i]] = i;

    for (int j = 0; j < n; ++j)
      for (int k = j + 1; k < n; ++k) {
        const int ai = arr[k] - arr[j];
        if (ai < arr[j] && numToIndex.count(ai)) {
          const int i = numToIndex[ai];
          dp[j][k] = dp[i][j] + 1;
          ans = max(ans, dp[j][k]);
        }
      }

    return ans;
  }
};
  • code (explanation):
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class Solution {
 public:
  int lenLongestFibSubseq(vector<int>& arr) {
    // Only get the value of size() or length() when we use it twice or more times.
    // Remember to add `const` whenever possible.
    const int n = arr.size();
    int ans = 0;
    vector<vector<int>> dp(n, vector<int>(n, 2));
    unordered_map<int, int> numToIndex;

    for (int i = 0; i < n; ++i)
      numToIndex[arr[i]] = i;

    for (int j = 0; j < n; ++j)
      for (int k = j + 1; k < n; ++k) {
        const int ai = arr[k] - arr[j];  // Add `const`.
        if (ai < arr[j] && numToIndex.count(ai)) {
          const int i = numToIndex[ai];  // Add `const`.
          dp[j][k] = dp[i][j] + 1;
          ans = max(ans, dp[j][k]);
        }
      }

    return ans;
  }
};

Boundary Conditions

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// Linked-List
if (l1 == nullptr && l2 == nullptr) { ... }
if (l1 != nullptr || l2 != nullptr) { ... }

// String
if (str.empty()) { ... }
if (str.length() <= 2) { ... }

// Vector
if (vec.size() <= 2) { ... }

Return Value

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return ans;
return {};

Data Structures

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// C++
unordered_set<string> seen;
unordered_map<char, int> count;
vector<int> count;
stack<char> stack;
queue<TreeNode*> q;
deque<TreeNode*> minQ;
auto compare = [](const ListNode* a, const ListNode* b) {
  return a->val > b->val;
};
priority_queue<ListNode*, vector<ListNode*>, decltype(compare)> minHeap(compare);

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// Java
Set<String> seen = new HashSet<>();
Map<Character, Integer> count = new HashMap<>();
int[] count = new int[n];
Deque<Character> stack = new ArrayDeque<>(); // Do not use `Stack`.
Queue<Integer> q = new LinkedList<>();
Deque<Integer> minQ = new ArrayDeque<>();
Queue<ListNode> minHeap = new PriorityQueue<>((a, b) -> a.val - b.val);

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# Python
seen = set()
count = {}
count = collections.defaultdict(int)
count = collections.defaultdict(list)
count = collections.Counter()
minQ = collections.deque([root])
stack = []
minHeap = []

Two Pointers / Sliding Windows

  1. Always prefer to one character to represent index variables.
  2. Use i, j, k in the loop, in that order.
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int i = 0;
for (const int num : nums) { ... }

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for (int i = 0, j = 0; i < n; ++i) { ... }

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int k = 0;
for (int i = 0; i < n; ++i)
  for (int j = i; j < n; ++j) { ... }

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int l = 0;
int r = nums.size() - 1;

Union Find

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class UnionFind {
 public:
  UnionFind(int n) : count(n), id(n), rank(n) {
    iota(begin(id), end(id), 0);
  }

  void unionByRank(int u, int v) {
    const int i = find(u);
    const int j = find(v);
    if (i == j)
      return;
    if (rank[i] < rank[j]) {
      id[i] = id[j];
    } else if (rank[i] > rank[j]) {
      id[j] = id[i];
    } else {
      id[i] = id[j];
      ++rank[j];
    }
    --count;
  }

  int getCount() const {
    return count;
  }

 private:
  int count;
  vector<int> id;
  vector<int> rank;

  int find(int u) {
    return id[u] == u ? u : id[u] = find(id[u]);
  }
};

Graph / Tree

  • If a graph has a clear tree structure, we name it a tree. Otherwise, we name it a graph.
  • Always use (u, v) to represent an edge regardless what is stated in the problem.
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vector<vector<int>> graph(n);

for (const vector<int>& edge : edges) {
  const int u = edge[0];
  const int v = edge[1];
  graph[u].push_back(v);
  graph[v].push_back(u);
}

Dijkstra

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void dijkstra(const vector<vector<pair<int, int>>>& graph, int src) {
  vector<int> dist(graph.size(), INT_MAX);
  using P = pair<int, int>;  // (d, u)
  priority_queue<P, vector<P>, greater<>> minHeap;

  dist[src] = 0;
  minHeap.emplace(0, src);

  while (!minHeap.empty()) {
    const auto [d, u] = minHeap.top();
    minHeap.pop();
    for (const auto& [v, w] : graph[u])
      if (d + w < dist[v]) {
        dist[v] = d + w;
        minHeap.emplace(dist[v], v);
      }
  }
}
  1. Always prefer to one character to represent index variables.
  2. Always prefer to use [l, r) pattern.
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int l = 0;
int r = nums.size();

while (l < r) {
  const int m = l + (r - l) / 2;
  if (f(m))
    l = m + 1;  // new range [m + 1, r)
  else
    r = m;  // new range [l, m)
}

return l;

ListNode

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// Allocate the memory on the stack instead of the heap.
ListNode dummy(0);

ListNode* curr;
ListNode* prev;
ListNode* next;

ListNode* slow;
ListNode* fast;

ListNode* head;
ListNode* tail;

ListNode* l1;
ListNode* l2;

2D Vector / 2 Strings

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// 2D Vector
const int m = matrix.size();
const int n = matrix[0].size();

// If there're two strings.
const int m = str1.length();
const int n = str2.length();

// If there's only a string.
const int n = str.length();

Traversing

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// vector<int> nums;
for (int i = 0; i < nums.size(); ++i) { ... }
for (const int num : nums) { ... }

// vector<string> words;
for (const string& word : words) { ... }

// string str;
for (int i = 0; i < str.length(); ++i) { ... }
for (const char c : str) { ... }

// unordered_set<int> numsSet;
for (const int num : numsSet) { ... }

// structured binding in C++17
// unordered_map<char, int> map;
for (const auto& [key, value] : map) { ... }

// ListNode* head;
for (ListNode* curr = head; curr; curr = curr->next) { ... }

Miscellaneous

  1. Always prefer to use str.length() over str.size().
  2. Always use camelCase nomenclature when not listed above.
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// C++
int currNum;
int maxProfit;
TreeNode* currNode;
  1. When there's confliction in expression and function or reserved key word:
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// C++
mini, std::min()
maxi, std::max()

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# Python
mini, min
maxi, max
summ, sum
  1. When there are two maps/stacks, use meaningful names.
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// C++
unordered_map<char, int> countA;
unordered_map<char, int> countB;
  1. When we need to count something, use sum, count and total, in that order.
  2. Initialize vector with 0 or false implicitly.
  3. constexpr is used if possible.
  4. const is used if possible.
  5. const auto is used when we iterate through a unordered_map or map.
  6. Use & whenever possible except int and char because reference typically takes 4 bytes, while int takes 2/4 bytes and char takes 1 byte.
  7. Prefer to name variables in a "adjective + noun" order. For example, maxLeft is better than leftMax.
  8. If a block is really small, for example, before a bfs() call, sometimes we don't add extra blank lines. This is not a hard rule.